The present invention relates to a wheel suspension for steerable rear wheels of motor vehicles, especially passenger motor vehicles, equipped with a front wheel steering system, which includes one wheel carrier for each rear wheel that is pivotally connected with the vehicle body or the like about a steering axis of rotation by way of a longitudinal guide member extending at least approximately in the vehicle longitudinal direction and cross guide members extending at least approximately in the vehicle transverse direction, of which one cross guide member is displaceable in the vehicle transverse direction by an adjusting motor for purposes of steering.
In a known wheel suspension of this type for front wheel driven vehicles, one spring leg and two cross guide members arranged parallel to one another and one behind the other with a spacing from each other are provided at each rear wheel-wheel carrier, of which the forward one is pivotally connected with its outer end at the lower end of the spring leg and with its inner end at the vehicle body whereas the rear one is connected with its outer end by way of a steering arm also with the lower end of the spring leg and with its inner end with a thrust or connecting rod actuatable hydraulically or by means of an electric motor (DE-OS 35 07 098). An inclined positioning of the rear wheel results with this prior art wheel suspension in case of a steering deflection. The build up of a lateral force which follows in time the adjustment of the slip angle, reaches its magnitude corresponding to the slip angle only after a certain time interval after the initiation of the steering operation. With the relatively small deflection angles which are encountered in rear wheel steering systems particularly at higher driving velocities, at which the lateral force should be available rapidly, the lateral force build-up desired for the lateral shifting of the vehicle, requires considerable time.
In another known wheel suspension of the aforementioned type, a spring leg and two cross guide members parallel to one another and arranged at a distance from one another one behind the other, are provided at each rear wheel-wheel carrier, which cross guide members are pivotally connected with each respective outer end at the wheel carrier and with its inner end at the vehicle body and of which the rear one is not changeable in its length whereas the front one is changeable in its length for steering purposes by means of a hydraulic telescoping unit integrated into the same (JP-OS 60 92 977). An inclined positioning of the rear wheel also results with this prior art wheel suspension during a steering deflection which is followed only in time by the build-up of a lateral force.
In a further known wheel suspension for steerable rear wheels of vehicles equipped with front steering and rear wheel drive (DE-OS 36 23 885), a longitudinal guide member and two cross guide members arranged at least approximately parallel to one another and arranged with a spacing from one another one on top of the other, engage at each rear wheel-wheel carrier (DE-OS 36 23 885). The joints, by way of which the two cross guide members are connected at their outer ends with the wheel carrier, define the steering axis of rotation which rises forwardly. The inner ends of the two cross guide members and the forward end of the longitudinal guide member are secured at an auxiliary frame or subframe. The rear end of the longitudinal guide member is rigidly connected with the rear wheel-wheel carrier. The forward area of the longitudinal guide member is displaceable in the vehicle transverse direction with the aid of an adjusting motor constructed as hydraulic cylinder. A steering deflection of a rear wheel is effected by pivoting the longitudinal guide member about the steering axis of rotation. The inclination of the steering axis of rotation thereby causes a displacement of the rear wheel which acts opposite the build-up of a lateral force in the direction of the desired lateral shift of the vehicle.
A wheel suspension for steerable rear wheels of motor vehicles equipped with front wheel steering is also known (DE-OS 36 09 656) in which the rear wheel-wheel carrier is guided by a longitudinal guide member and an inclined guide member. The inclined guide member, on which is supported a spring leg, is secured with its end on the body side at a tubularly shaped transverse member which is secured at the vehicle body by way of elastic bearings. Inside of the tubularly shaped transverse member, a toothed rack is supported displaceable in the vehicle transverse direction which at its end protruding out of the tubularly shaped transverse member is connected with the forward end of the longitudinal guide member. At their ends on the wheel side, the longitudinal guide member and the inclined guide member are pivotally connected with each other by a joint pin arranged to the rear of the wheel center. The axis of this joint pin, which forms the steering axis of rotation, is arranged inclined rising toward the rear. The steering of the rear wheel is effected in that the toothed rack is displaced in the vehicle transverse direction and the longitudinal guide member is thereby pivoted about the joint pin.
The present invention is concerned with the task to provide a wheel suspension of the aforementioned type in which a rapid build-up of lateral force for the desired lateral shift of the vehicle is assured by simple means in case of a steering deflection of the rear wheels unidirectional with the steering deflection of the front wheels.
The underlying problems are solved according to the present invention in that the joints, by way of which two cross guide members are connected with the wheel carrier, define the steering axis of rotation and the latter is arranged in a plane extending in the vehicle longitudinal direction inclined with respect to a vertical vehicle transverse plane in such a manner that its rises toward the rear. It is achieved thereby that during the drive, especially at higher velocities, with unidirectional steering deflection of the front and of the rear wheels not only a rear wheel inclined positioning, but also a change of the wheel camber results which is effective in the sense of a vehicle displacement or shifting toward the side predetermined by the steering deflection. This leads to a relatively rapid build-up of a lateral force directed correctly from a shifting point of view.
The rapid lateral force build-up to be achieved by the present invention has as a consequence that a desired lateral shift of the vehicle can be achieved already with relatively small deflection angles. The deflection angles which are to be established maximally, can thereby be very small. This leads to a slight space requirement. The space between the rear wheels can therefore be utilized far-reachingly for other purposes, for example, as luggage space.
By reason of the rearwardly rising steering axis of rotation, i.e., inclined for the forward driving direction about a caster angle, in case of unidirectional steering deflection of the rear wheels of the two vehicle sides from the straight position, the camber of the rear wheel on the outside of the curve shifts in a negative direction, i.e., the upper part of the wheel is displaced inwardly, and the camber of the rear wheel on the inside of the curve shifts in a positive direction, i.e., the upper part of the wheel shifts outwardly. Unidirectional lateral displacements result therefor at the rear wheels of the two vehicle sides and therewith unidirected lateral forces which with a unidirectional steering deflection of the front and of the rear wheels act in the direction in which the vehicle is to be shifted laterally by the steering.
The steering axis of rotation may extend through the wheel center as viewed in side view; however, it may also be arranged at a distance therefrom. Depending on the arrangement of the steering axis of rotation, the point in which the steering axis of rotation extends through the tire contact surface (road surface) may be located in front or to the rear of the tire contact point or may coincide with the same. In order to obtain favorable driving conditions for the generally predominantly forward drive, it is advantageous to always arrange the steering axis of rotation in such a manner that a caster of the rear wheel is provided for the forward driving direction. The latter effects the endeavor of the rear wheels to adjust themselves automatically into the straight position and to maintain the same. Additionally, it counteracts any possible flutter tendency of the rear wheels. The magnitude of the caster offset can be determined in a known manner.
A particularly favorable construction is realized according to the present invention if three cross guide members are pivotally connected at the wheel carrier, a triangular arrangement is provided for the joints of the cross guide members at the wheel carrier in which either two joints are below the wheel center and the third joint above the wheel center or two joints are above the wheel center and the third joint is below the wheel center and the respective third joint is arranged between the two other joints as viewed in vertical projection, and the two joints of the cross guide members fixed in the vehicle transverse direction define at the wheel carrier the steering axis of rotation. This construction is particularly compact and advantageous also for the driven rear wheels.